Color is the perceptual result of light in the visible region of the spectrum (wavelengths approximately in the region of 400 nanometer (nm) to 700 nm) incident upon the retina of the human eye. The human retina has three types of color photoreceptors or cone cells, which respond to incident radiation with somewhat different spectral response curves. Because there are exactly three types of color photoreceptors, three numerical components are necessary and sufficient to describe a color, providing that appropriate spectral weighting functions are used. One description of color uses the representation “RGB”, or “RGB color space”, and refers to colors red (R), blue (B) and green (G). The red, blue and green colors include the color bands that conventional solid-state electronic cameras capture; these colors also approximately represent colors as viewed by humans. It is a challenge for the designers of digital imagers to achieve solutions that provide images almost equivalent to human vision.
Another description of color includes “YUV,” a color encoding system used for analog television worldwide (NTSC, PAL and SECAM). When color television (TV) signals were developed in the 1950s, YUV was used to encode colors in order to allow black and white TVs to continue to receive and decode monochrome signals from TV signals, while color sets would decode both monochrome and color signals. The Y in YUV represents “luma” which is brightness, or lightness, and black and white TVs decode only the Y part of the signal. The U and V in YUV represent color (chroma) information and are “color difference” signals of blue minus luma (B−Y) and red minus luma (R−Y). The terms luma and chroma are often interchanged with luminance and chrominance, respectively, as the difference between these terms is a minor difference having to do with use of gamma corrected or linear pixel signals used in the calculations.
A conventional video camera uses a process referred to as “color space conversion” to convert the RGB data captured by its solid-state sensor into either composite analog signals (YUV) or component versions (analog YPbPr, or digital YCbCr). The difference between YCbCr and RGB is that YCbCr represents color as brightness and two color difference signals, while RGB represents color as red, green and blue. In YCbCr, the Y represents the brightness (luma), Cb represents blue minus luma (B−Y) and Cr represents red minus luma (R−Y). It is desirable in digital cameras to eliminate RGB conversion and accomplish direct detection of digital YCbCr signals within the image sensor. Direct detection of YCbCr within the image sensor eliminates the need for RGB conversion, and may provide better color rendition and increase image sensor dynamic range. While RGB may be the most commonly used basis for color descriptions, it has the negative aspect that each of the coordinates (red, green, and blue) is subject to luminance effects from the lighting intensity of the environment
Composite analog signals (YUV) (and analog YPbPr or digital YCbCr) reduce transmission bandwidth compared to RGB because the chroma channels (B−Y and R−Y) carry only half the resolution of the luma. YUV is not compressed RGB; rather, Y, B−Y and R−Y are the mathematical equivalent of RGB. Moving Picture Expert Group (MPEG) compression, which is used in digital video disks (DVDs or, alternatively, digital versatile disk), digital TV and video compact disks (CDs), is coded in YCbCr. Furthermore, digital camcorders (e.g., MiniDV, digital video (DV), Digital Betacam, etc.) output YCbCr over a digital link such as FireWire. The reason for using YCrCb signals is that the human eye is less sensitive to chrominance than luminance. Compression algorithms can take advantage of this phenomenon and subsample the values of Cb and Cr without significant visual degradation of the original color signal.
Despite improvements in solid-state image sensor and digital camera technology, the basic detection mechanism for color cameras is RGB and the detected signal requires reformatting to YCrCb to separate the RGB signals into luminance and chrominance data sets for image compression and resultant image transmission or image data storage. Consequently, there is a need for a digital camera with direct luminance and chrominance detection to eliminate reformatting of RGB signals.